Which key parameters need to be strictly controlled during the production process to ensure the quality of the final graphitized petroleum coke?

In the production process of graphitized petroleum coke, it is essential to strictly control the following key parameters from raw material selection, pretreatment, graphitization process to post-treatment to ensure the quality of the final product:

I. Raw Material Selection and Pretreatment

Sulfur Content

• Control Standard: The sulfur content of raw petroleum coke should be ≤0.5%. High-sulfur coke can cause gas expansion during graphitization, leading to product cracking.
• Impact: Every 0.1% reduction in sulfur content decreases product cracking rate by 15%-20% and reduces resistivity by 5%-8%.

Ash Content

• Control Standard: Ash content should be ≤0.3%, with primary impurities being metal oxides such as iron, silicon, and calcium.
• Impact: Every 0.1% increase in ash content raises product resistivity by 10%-15% and decreases mechanical strength by 8%-10%.

Particle Size Distribution

• Control Standard: Granular coke should account for ≥80%, while powdered coke (particle size <0.5 mm) should be ≤20%.
• Impact: Excessive powdered coke can lead to caking during calcination, affecting volatile matter removal; improved uniformity of granular coke reduces graphitization energy consumption by 5%-10%.

Calcination Process

• Temperature: 1200-1400°C for 8-12 hours.
• Function: Removes volatile matter (from 8%-15% to <1%) and increases true density (from 1.9 g/cm³ to ≥2.05 g/cm³).
• Control Point: True density after calcination must be ≥2.08 g/cm³; otherwise, graphitization difficulty increases, and resistivity rises.

II. Graphitization Process

Temperature Control

• Core Parameter: 2800-3000°C, maintained for 48-72 hours.
• Impact:
  • Every 100°C increase in temperature enhances crystallinity by 5%-8% and reduces resistivity by 3%-5%.
  • Insufficient temperature (<2700°C) results in amorphous carbon residue, with product resistivity >15 μΩ·m; excessive temperature (>3100°C) may cause carbon structure damage.

Temperature Uniformity

• Control Standard: Temperature difference between furnace core and edge ≤150°C, with thermocouple spacing ≤30 cm.
• Impact: Every 50°C increase in temperature difference expands local resistivity variation by 10%-15% and decreases product yield by 5%-8%.

Heating Rate

• Control Standard:
  • 25-800°C stage: ≤3°C/h (to prevent thermal stress cracking).
  • 800-1250°C stage: ≤5°C/h (to promote ordered carbon structure formation).
• Impact: Excessive heating rates cause product volume shrinkage exceeding 15%, leading to cracking.

Protective Atmosphere

• Control Standard: Nitrogen flow rate of 0.8-1.2 m³/h, or use of argon/vacuum environment.
• Function: Prevent oxidation and reduce impurity content (e.g., oxygen content decreases from 0.5% to <0.1%).

III. Post-Treatment and Purification

Cooling Rate

• Control Standard: Slow cooling rate ≤20°C/h after graphitization.
• Impact: Rapid cooling causes residual thermal stress, reducing product thermal shock resistance by 30%-50%.

Crushing and Screening

• Control Standard: Particle size D50 controlled at 10-20 μm, with surface coating (e.g., pitch or chemical vapor deposition) thickness uniformity ≤5%.
• Function: Optimizes particle morphology and increases product bulk density (from 0.8 g/cm³ to ≥1.2 g/cm³).

Purification Treatment

• Halogen Purification: Cl₂ gas reacts at 1900-2300°C for 24 hours, reducing impurity content to ≤50 ppm.
• Vacuum Purification: Maintained at 10⁻³ Pa vacuum for 50 hours, achieving total impurity content ≤10 ppm (for high-end applications).

IV. Summary of Key Control Points

V. Technological Trends and Optimization Directions

Ultrafine Structure Control: Develop 0.1-1 μm coke powder preparation technology to enhance isotropy and reduce resistivity to <5 μΩ·m.
Smart Manufacturing Systems: Implement digital twin-based temperature field dynamic control systems to increase yield to 95%.
Green Processes: Use hydrogen as a reducing agent to reduce CO₂ emissions; adopt waste heat recovery technology to lower energy consumption by 10%-15%.

By strictly controlling these parameters, graphitized petroleum coke can achieve a carbon content ≥99.9%, resistivity of 5-7 μΩ·m, and a thermal expansion coefficient of 1.5-2.5×10⁻⁶/°C, meeting the demands of high-end industrial applications.